We often use an infinitive to talk about a person’s purpose.
- He rose to speak.
- She came to the city to look for work.
- He sat down to rest.
We can also use in order to or so as to.
- She came to the city so as to look for work.
So as to and in order to are more common before verbs like be, have, know etc.
- He raised his voice so as to be heard.
- He got up early in order to have enough time to pack.
Before a negative infinitive, we normally use so as or in order.
- I am leaving now so as not to be late. (NOT I am leaving now not to be late.)